This application is directed to a method and system for producing low-noise acoustical impulse responses and more particularly to a method and system for producing low-noise acoustical impulse responses at high sampling rates for use in acoustical systems.
A well-established approach for measuring impulse responses (IRs) of acoustical systems at sampling rates of either 44.1 kHz or 48 kHz involves the use of an exponential sine sweep (ESS) up to the Nyquist frequency. However, some acoustical systems have responses that extend beyond 24 kHz, and using this conventional ESS approach presents problems.
If the conventional ESS approach is used to measure a system whose response extends beyond the Nyquist frequency, the anti-aliasing filter of the A/D converter will low-pass filter the system response with a cut-off near the Nyquist frequency, eliminating the response that exists above this point. This would manifest itself as a spurious (non-causal) pre-response in the measured IR. Therefore, in order to accurately characterize the entire system response, it is necessary to adopt higher sampling rates. Doing so would eliminate the pre-response, but poses the risk of damaging the transducers when attempting a sweep to a higher Nyquist frequency. This is especially true when using long-duration sweeps as is done in existing methods. Additionally, as the system response may now fall below the noise floor prior to the Nyquist frequency, high-frequency noise can potentially contaminate the measurement and yield a less than maximal signal-to-noise ratio.
Abrupt termination of the sweep before the Nyquist frequency introduces artifacts such as an end-of-sweep “pop” in the transducer that corrupts the measurement and may damage the transducers. This issue can be prevented by applying a fade-out to the end of the sweep as is done in existing methods. However, this solution is only viable when employing the time-reversed sweep inversion approach derived by Angelo Farina and described in the article entitled “Simultaneous Measurement of Impulse Response and Distortion with a Swept-Sine Technique,” presented at the AES 108th Convention, February 2000, since the exact, frequency-domain inverse of the faded-out sweep will cause excessive high-frequency noise amplification. However, the time-reversed sweep inversion approach produces a pre-response if the sweep does not cover the entire system response.
A critical step in such impulse response interpolations is locating impulse response onsets for time-alignment, a task which becomes difficult in the presence of pre-responses. Furthermore, it has been found that pre-responses can be audible as described by Peter G. Craven in “Antialias Filters and System Transient Response at High Sample Rates,” J. Audio Eng. Soc., 52(3):216-242 (2004), and so their suppression is desirable.
It is therefore an object of the present invention to provide a method and system of measuring low-noise acoustical impulse responses at high sampling rates that enable low noise measurement without non-causal pre-response contamination.